1. Field of the Invention
The present invention relates to a magnet pump, a method of producing the magnet pump and a nuclear reactor equipment which incorporates the magnet pump.
2. Description of the Related Art
In general, a magnet pump does not have any shaft seal portion so that it can suitably be used for the purpose of transfer of chemical waste liquids, slurries and so forth.
FIG. 6 illustrates the basic construction of a known magnet pump.
This magnet pump has a magnetic coupling to which the torque of a motor is transmitted through a drive shaft 1. The magnet coupling has an outer ring 2 having a magnet 4 and an inner ring 3 having a magnet 5. The torque of the drive shaft 1 causes the outer ring 2 to rotate, which in turn causes the inner ring 3 to rotate as a result of attractive and repulsive forces acting between the magnets 4 and 5, so that a driven shaft 10 connected to the inner ring 3 is rotated. The driven shaft 10 in turn drives an impeller 11 connected thereto, so that a liquid is sucked through a suction port 13 and is discharged from the discharge port 14. A can 12 serves to isolate the motor part including the outer ring 2 and the liquid-contact part including the inner ring 3 from each other.
Thus, the magnet pump is devoid of any shaft seal portion so that it can suitably be used in handling a chemical liquid which requires specific leak prevention measure, as well as slurries which inevitably cause grinding or wear of sliding parts.
The magnet pump also can suitably be used as a coolant circulation pump of a nuclear reactor. In particular, the use of a magnet pump in the primary cooling system eliminates any risk of external leak of coolant which radiates radioactive rays such as a .gamma. ray. In addition, the time required for a periodical inspection can remarkably be shortened because only the liquid-contact part of the pump needs inspection, so that the time of exposure of the inspectors to a radioactive atmosphere is shortened, thus enhancing safety.
This type of magnet pump is disclosed, for example, in "Piping Technology", pp 55-57, Jan. 5, 1987.
Unfortunately, however, the known magnet pump has not been considered for transmission of a large torque, which is a fundamental requirement for pumps. Thus, it has been quite difficult to realize a large-scale magnet pump capable of operating at a torque level of 15 kg.multidot.m or greater required in nuclear reactor coolant circulation pump and other pumps.
A greater scale of the magnet pump essentially requires a greater torque capacity of the magnet coupling, i.e., to increase the magnetic power of the magnet.
Conventionally, ferrite or a rare earth magnet, in particular samarium cobalt (referred to as "SmCo", hereinafter) magnet, has been used as the magnets of the magnet couplings of magnet pumps. SmCo has a large value of the maximum energy product (BH)max which is specifically useful for large-scale magnet couplings. However, there still exists the following problem which has to be overcome.
Small-capacity magnet couplings are produced by the following processes: namely, a non-magnetized magnet is fixed to the inside of, for example, an inner ring yoke. This assembly is then placed in a mold which is then charged with a resin of at 150.degree. to 250.degree. C. Then, the magnet is magnetized by means of a magnetizing yoke. The following problems are encountered when this known process is applied to the production of a large-scale magnet coupling.
(1) Quite a large-size magnetizing equipment is necessary.
(2) It is difficult to separate the magnet from the magnetizing yoke after the magnetization.
For these reasons, there has been a practical limit in the size of the magnet coupling.
In another process, a magnetized magnet is bonded to an inner or outer ring yoke and then resin-molding is conducted within a mold. In this process, a problem is encountered in that the separation of the resin-molded magnet from the mold is difficult due to magnetic attraction between the magnet and the mold metal.
A greater capacity of magnet coupling essentially requires greater maximum energy product (BH).sub.max of the magnet. Such large maximum energy product is obtained with a neodymium-iron type magnet, as well as with an SmCo magnet mentioned before. Properties of samarium-type magnet and neodymium-type magnet are shown in the following Table 1.
TABLE 1 ______________________________________ Properties of samarium-type magnet and neodymium-type magnet (Extracted from Denpa Shinbun, page 24, Sept. 10, 1987) Sm magnet Nd magnet ______________________________________ Elements Sm, Co, Fe, Cu Nd, Fe, B and and other other additives additives Magnetic 16.about.32 MGOe 27.about.37 MGOe characteristic (BH).sub.max Br 8.2.about.11.6 KG 10.2.about.12.6 KG iHc 6.2.about.20.0 kOe 11.0.about.20.0 kOe Reversible 1.05 1.05 magnetic permeability Reversible -0.03.about.-0.04%/c -0.12.about.-0.13%/c temperature change Br Curie point 800.degree. C. 320.degree. C. Cracking tendency fragile and easy hard and not to crack easy to crack Surface rusting Comparatively Comparatively small rusting large rusting tendency tendency ______________________________________
In general, the coolant circulation system of a nuclear reaction incorporates a plurality of circulation pumps, some of which are installed close to the nuclear reactor. Such circulation pumps are required to transport coolant which has high neutron beam density. Neutron beams makes elements such as Co radioactive, posing problems in regard to degradation of properties of the magnet and safety during inspection.
This gives a rise to the demand for a magnet which does not contain any radioactive element, and such a requirement is met by a neodymium-iron type magnet.
Unfortunately, however, this magnet exhibits an inferior corrosion resistance, so that a surface coating is essentially required. Furthermore a new sealing method has to be developed because the characteristic of the magnet tends to be degraded as a result of a temperature rise.
Thus, in order to obtain a large-capacity magnet coupling which is essential for producing a large-size magnet pump, in particular a coolant circulation pump of a nuclear reactor, it is necessary that a method is developed which enables an easy resin-molding of magnets of a magnet coupling without impairing the characteristics of the magnets.